Security is an important concern in the realm of documents and digital production and/or reproduction of same. Known digital image printing/copying systems produce documents of such high quality that a need has been identified to prevent effective printing/copying of certain documents such as high-value printed items including tickets, financial instruments, security passes, and the like. Known techniques include printing the original document in a manner such that it includes a “watermark,” using only conventional paper and toner or ink. Two such watermarking techniques have been developed and are known to increase document security, i.e., to limit counterfeit printing and copying: (i) fluorescence marks; and (ii) correlation marks.
Methods and systems are known for including fluorescence marks in printed documents using conventional papers (e.g., ordinary “copy paper”) and ordinary inks/toners (e.g., CMYK ink/toner), specifically by using metameric colorant mixtures. Under ambient (non-ultraviolet) or visible lighting conditions (e.g., wavelengths of about 400-700 nanometers (nm)), two different colorant mixtures that are printed on respective regions of the paper provide a printed document region that appears substantially uniform in color. Under ultraviolet (UV) lighting or radiation (e.g., wavelengths shorter than about 400 nm), these colorant mixtures exhibit different UV absorption and, thus, suppression of fluorescence of the optical brightening agents used in printing/copying papers such that regions printed with the colorant mixture that suppresses less of the substrate fluorescence appear as a lighter regions while the surrounding areas printed with the colorant mixture that strongly suppresses substrate fluorescence appear as a darker regions. These contrast variations under UV lighting are used to create watermark patterns, e.g., numbers, letters, symbols, shapes.
An example of this is shown in FIG. 1, wherein a colorant mixture “B” is selected and applied to patch area BP, which here is arranged in this example as the alphanumeric symbol “0”. Further, a colorant mixture “A” is selected and applied to patch area AP arranged here in substantially close spatial proximity to patch area BP, and thereby providing a background around patch area BP. Both colorant mixture A and colorant mixture B are comprised of a suitably selected colorant or colorant mixtures, but colorant mixtures A and B are different mixtures. Each colorant mixture A or B may be, for example, either a single CMYK colorant or any mixture of CMYK colorants. In the illustrated example, colorant mixture A will be selected so as to provide higher UV absorption (greater substrate fluorescence suppression) than that selected for colorant mixture B. The colorant mixtures A and B will also be selected to match each other closely in their average color and luminance under ambient light. As shown at UVL in FIG. 1, under UV lighting conditions, patch BP will appear brighter as compared to patch AP, due to the relatively limited suppression of the fluorescence of the optical brightening agents in the paper substrate as compared to the patch AP, thus forming a watermark W. In contrast, under ambient light conditions as shown at AML, patches AP,BP are at least substantially indistinguishable. By way of example an approximate 50% grayscale gray colorant mixture may be realized with a halftone of black (K) colorant only and used for colorant mixture B to print patch BP. This may then be color-matched against a colorant mixture A comprising a high amount of yellow (Y) mixed with enough cyan (C) and magenta (M) to yield a similar approximate 50% grayscale gray colorant mixture A which is used to print the patch AP. With the given high content of yellow colorant in colorant mixture A, this colorant mixture will provide much higher absorption of UV or suppression of native substrate fluorescence as compared to the patch BP, so that under UV lighting conditions, the patch BP will be readily apparent as a watermark W1. The two colorant mixtures will appear quite nearly identical “gray” under normal viewing or “ambient’ illumination as shown at AML in FIG. 1. Thus, when a document including such a fluorescence mark is subjected to UV illumination, the watermark W1 is revealed. A printed “look-alike” document or mere photocopy will not include the watermark. Those of ordinary skill in the art will recognize that the difference in substrate fluorescence suppression as between colorant mixtures A and B results primarily from the fact that the colorant mixture B (single black colorant in the current example) requires less substrate coverage for a desired gray level printed region as compared to colorant mixture A (a combination of cyan, magenta, yellow colorants in the current example) that requires more ink/toner coverage of the substrate to achieve a corresponding matching gray level. Thus, a high level of yellow colorant in mixture A leads to more overall substrate coverage (greater UV fluorescence suppression) because higher amounts of cyan and magenta are required in colorant in mixture A to match the printed gray level of the colorant mixture B. Additional details and variations relating to fluorescence marks are disclosed in U.S. patent application Ser. No. 11/382,897 filed May 11, 2006 in the name of Raja Bala and Reiner Eschbach and entitled “Substrate Fluorescence Mask for Embedding Information in Printed Documents” and U.S. patent application Ser. No. 11/382,869 filed May 11, 2006 in the name of Raja Bala and Reiner Eschbach and entitled “Substrate Fluorescence Pattern Mask for Embedding Information in Printed Documents” and the disclosures of both these applications are hereby expressly incorporated by reference into the present specification.
As noted, a correlation mark is another known watermarking technique. By way of example, as shown in FIG. 2, a checkerboard halftone pattern HP is printed on paper by a regular 45-degree cluster halftone screen and has 13 columns of halftone cells. Another checkerboard halftone pattern HP′ covering an equal spatial area overall is printed on paper by a halftone screen having four slightly stretched columns C5-C8 in the middle section, such that the halftone pattern HP′ has only 12 columns over the same spatial width as the pattern HP. The patterns HP and HP′ are perceived as the same gray level to a human observer and the phase shift is substantially hidden under normal image viewing conditions. In the pattern HP′, due to the pitch difference between the regular columns C1-C4,C9-C12 and the slightly stretched columns C5-C8, the phase, which represents the transition between white and black, has changed from “in phase” with the pattern HP, (0 radian phase shift), for columns C1-C4, to “opposite phase” (π radian phase shift) for columns C9-C12, with varying, increasing phase shift for the stretched columns C5-C8. If a transparency key K is printed with the halftone pattern HP and overlaid with the paper-printed halftone pattern HP as shown at K+HP, the result is unchanged relative to the original printed pattern HP because the key K and printed pattern can be registered, solid-on-solid, blank-on-blank. In contrast, when the key K is overlaid with the phase-shifted pattern HP′ as shown at K+HP′, the original printed checkerboard pattern HP′ gradually disappears in proportion to the phase difference between the pattern HP′ and the key K moving from left to right in the image, until the checkerboard pattern HP′ is completely black (or other solid color) where the pattern HP′ and key K are opposite phase. The average reflectance is higher in the registered regions C1-C4 and lower in the partially phase shifted regions C5-C8 and still lower in the opposite phase regions C9-C12. Therefore, if a portion of an image is encoded with π radian phase-shifted halftone cells and immediately surrounding these π radian phase-shifted halftone cells are halftone cells that are 0 radian phase-shifted, by overlapping a checkerboard pattern transparency key of the same spatial frequency, a high contrast watermark W2 can be defined by the low reflectance areas where the key and halftone pattern are out of phase. Accordingly, a document can be printed in this manner to include a watermark that is completely or at least substantially undetectable until a corresponding transparency key is overlaid to reveal the phase-shifted halftone regions. Correlation marks are disclosed, e.g., in U.S. Pat. No. 6,252,971 to Wang entitled “Digital Watermarking Using Phase-Shifted Stoclustic Screens” which is also hereby expressly incorporated by reference into the present disclosure.
While both fluorescence marks and correlation marks are highly effective, a need has been identified for a watermark that is less apparent under ambient lighting conditions and more apparent during a security check of the document. Accordingly, the present development combines the principles of fluorescence marks and correlation marks to achieve this goal.